My opinion is to have patience with the weekend away. Try to leave it in his hands... he did say he wanted to take a weekend away didn't he? He just has a busy schedule.
About the "friend." I think you should accept the invitation. Treat the friend well. Don't go into the situation letting your opinion of him affect your actions. The more you don't want your H to spend time with this friend, naturally, he will be more inclined to spend time with him.
Let him figure out for himself that this guy is not a "good" friend.
I have gone through periods in my life where I hung out with people and then realized that I was not really that type of person to be involved with the group I was hanging with. But if someone would have said something to me about it at the time, I probably would have gotten defensive and it would have taken me longer to stop.
And don't worry why he invited you. He invited you. If you want to go... go and have a good time!
